RFR: 8279063: Consolidate push and push_if_necessary in PreservedMarks

[Albert Mingkun Yang]

Simple change of cleaning up `PreservedMarks` API.

Test: hotspot_gc

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Commit messages:
 - perserve-marks

Changes: https://git.openjdk.java.net/jdk/pull/6909/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6909&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279063
  Stats: 17 lines in 4 files changed: 1 ins; 9 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6909.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6909/head:pull/6909

PR: https://git.openjdk.java.net/jdk/pull/6909